The present invention relates to electrically heated catalytic converter modules for automotive exhaust gas treatment systems. More particularly, the invention relates to an improved design and assembly method for an electric heater enclosure for such treatment.
A number of exhaust system designs are being considered to meet targeted exhaust emissions requirements for ultra-low emission vehicles (ULEV requirements). Among the designs being considered are those employing an electrically powered fluid heater positioned upstream and adjacent to an oxidation catalyst unit. The electric heater heats the exhaust gas and catalyst unit during initial or "cold-start" engine operation, thereby advancing the onset of catalytic oxidation and reducing the emission of hydrocarbons (unburned fuel) otherwise emitted during that operation.
Attempts to contain and protect these heaters in the exhaust stream have involved approaches similar to those used to mount or "can" ceramic honeycomb catalyst substrates. These include the use of axially assembled enclosures such as described in U.S. Pat. No. 4,207,661 to Mase et al. Included in those enclosures are front and rear supporting members composed of a resilient material for supporting the catalytic converter substrate within the enclosure while shielding it from mechanical shocks.
U.S. Pat. Nos. 4,142,864 and 4,413,392 disclose the packaging of similar ceramic substrates by "stuffing" the substrates into cylindrical can segments, followed by the attachment of retaining rings and or conical end caps to the can segments. Axial assembly in the manner of the above patents is advantageous in that the number of components required to securely encase the catalytic substrate within the shielding metal container is relatively small, and in that tight axial constraint of the substrates within the enclosures is easy to achieve.
However, substantial difficulties relating to service life have been encountered in adapting the techniques of ceramic substrate canning to the canning of extruded metal honeycomb heaters used for the preheating of exhaust gases for subsequent catalyst treatment. This is largely because metal heater units, being composed of thin metal, are relatively ductile and thus somewhat more prone to vibration damage than ceramic honeycombs.
The durability requirements for metal heaters will be as stringent as those for ceramic honeycombs used as automotive catalyst supports. The enclosure system used must provide environmental and physical protection adequate to enable the heaters to meet government mandated standards for maximum allowed levels of non-methane hydrocarbons, CO, and nitrogen oxides for up to 100,000 miles of automobile use. Up to 50,000 engine starting cycles as well as severe thermal cycling, extreme temperatures, and high temperature vibration will be encountered by the heaters during this interval.
One previous approach to the containment of electrical honeycomb heaters, described in a co-pending, commonly assigned patent application for an "Axially Assembled Enclosure For Electrical Fluid Heater and Method", filed Aug. 30, 1994 by L. S. Rajnik et al., uses a two-piece enclosure wherein the heater is supported by compression between two halves of a steel enclosure welded together. These can halves are designed to firmly support and protect the honeycomb and associated insulating matting material from vibration damage in use.
While approaches such as above have been effective to substantially extend the service life of these heaters, they have not fully met the most stringent service life requirements, particularly under the more severe vibrational conditions of the hot exhaust environment.
It is therefore a principal object of the present invention to provide an improved metal honeycomb heater assembly useful in the treatment of engine exhaust gases which offers significant improvements in heater durability under conditions approximating the environment of use.
It is a further object of the invention to provide a canning method applicable to the canning of extruded metal honeycombs which provides a durably contained heater unit exhibiting high resistance to hot vibration damage over a prolonged period.
Other objects and advantages of the invention will become apparent from the following description.